United Nations human rights investigators said on Tuesday they had “reasonable grounds” to believe that limited amounts of chemical weapons had been used by government forces in Syria.

In their latest report, based on interviews with victims, medical staff and other witnesses, they said they had received allegations that both Syrian government forces and rebels had used the banned weapons, but that at least four instances related to their use by state forces.

“There are reasonable grounds to believe that limited quantities of toxic chemicals were used. It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator,” Paulo Pinheiro, who chairs the UN commission of inquiry, told a news conference in Geneva.

The commission examined four reported toxic attacks in March and April but could not determine which side was behind them.